Such X-ray sources have been employed in applications such as X-ray spectroscopy, X-ray lithography, radiobiology and X-ray microscopy, and an X-ray source of this kind has been described in considerable detail in a paper entitled "100 Hz KrF Laser-Plasma X-ray Source" presented at a conference entitled "Excimer Lasers and Applications III" at The Hague, Netherlands in 1991 and published in the Proceedings of SPIE--The International Society for Optical Engineering (SPIE Vol. 1503 Excimer Lasers and Applications III (1991), 391-405). As there described, an X-ray source comprises an excimer laser system which generates UV light pulses, at a repetition rate up to 100 Hz, which are focused on to an X-ray target. The target consists of a tape of suitable material, such as copper, steel or mylar, chosen in dependence on the desired frequency of the X-rays to be generated, and a tape transport mechanism which moves the tape so that each light pulse impinges on a fresh part of the tape, undamaged by previous pulses. Each light pulse, impinging on the tape, generates at the tape surface a highly localized volume of plasma which emits the desired X-rays but also has the effect of attenuating the UV light and shielding the tape from it, and thereby limiting the duration of the period in which X-rays are emitted. The incident UV light also causes debris to be ejected from the tape, and the debris tends to settle on surfaces of the optical system which delivers the UV light pulses to the tape and thus reduce the intensity of the incident light. As described in the above-mentioned paper, attempts to minimize these shielding effects include arranging a flow of helium gas across the part of the tape surface on which the UV light is incident, so as to sweep away and remove the debris, and assist in dissipating the plasma rapidly at the end of a pulse, and also to operate the system with the target not under highly reduced pressure but in a helium atmosphere at or approaching atmospheric pressure, which does not affect the emission of X-rays but has the effect of stopping and removing fast ions emitted by the plasma. In spite of such measures, however, it is found that in use of the known apparatus referred to the X-ray pulse generated by a pulse of UV light lasting 20 or 30 nanoseconds is generally limited to a duration of not more than 5 nanoseconds.
This shielding of the target from the incident UV laser light, in the known X-ray source referred to above, and heat loss from the expanding plasma, represent a severe limitation of the "conversion efficiency" (i.e. the ratio of X-ray energy to laser energy) of the apparatus, and a corresponding limitation on its average X-ray output power. These are factors which greatly affect the suitability of such apparatus for use in, particularly, X-ray lithographic work, for example in microcircuit production, where the highest possible average X-ray powers are required in order to minimize processing times.
It has been proposed (App. Phys. Lett. 55 (25), December 1989 and 71 (1), January 1992) to reduce the shielding effect, and improve the conversion efficiency and the average X-ray output power in such apparatus, by arranging that the laser light is emitted not in individual pulses with a pulse duration measured in tens of nanoseconds but in trains of ten to fifteen substantially shorter pulses, each pulse having a duration of about 0.10-0.15 nanoseconds, with the overall duration of the pulse train being about 20 to 30 ns. This proposal enables the conversion efficiency and average X-ray output power, when comparing a pulse train of 20 or 30 ns duration with a single pulse of equal length, to be increased significantly, by a factor of about three; but further improvements in these respects is required to make apparatus of this kind practical and competitive, and it is an object of this invention to provide such further improvements in a substantial degree.